Multistage amplifier



Aug. 15, 1932.

P. BORlAS 1,872,109

MULTISTAGE AMPLIFIER Filed July 5, 1929 i i l ln-wwa, N

5 HIM AAAllA I P01614770 =02) zzif INVENTOR ,Paud Bart/ad, BY

ATTORNEY Patented Aug. 16, 1932 UNITED STATES PAUL BORIAS, 0F PARIS,FRANCE MULTISTAGE AMPLIFIER Application filed July 5, 1929, Serial No.376,249, and in France July 5, 1928.

The present invent-ion relates to amplifiers, and, more particularly, tocoupling means for multi-stage amplifiers.

A multi-stage amplifier has been disclosed in application Serial No.254,988, filed February 17, 1928 which possesses novel characteristicsto be briefly referred to. The reactance of the plate circuit for thefrequency to be received is eliminated with the result that all reactiondue to the internal capacitance of the tube is prevented, inasmuch asthe plate is at an alternating current potential practically of zerovalue.

More specifically the amplifier arrangement in the said applicationcomprised a series resonant coupling between stages, the couplingincluding an inductance in series with a resistance and a capacity inshunt with the latter. As will be readily noted this arrangement doesnot transmit the higher frequencies and leaves a residue ofamplification for the lower frequencies.

Now, it is well known that attempts to insure high quality ofreproduction of audio or musicalfrequency currents is attended,

in the audio stages, with difiiculties of filtering out the radiofrequency currents which.

still exist at the output end of the detector tube.

detector tube and the first audio stage.

Another important object of the invention is to provide a couplmgarrangement between the audio stages which will successfully fiLer outresidual radio frequency currents which exist at the output ends ofpreceding stages. The invention will bebetter understood from thefollowing description by reference to the accompanying drawing in which,

Fig. 1 shows a multi-stage amplifier embodying a series resonantcoupling,

Fig. 2 shows a novel form of coupling arrangement to a succeeding audiostage,

Fig. 3 shows a graphic representation of t the operation of theamplifier shown in Fig. 2.

As stated above, there has been disclosed interposed between the plateand filament of tube 1.

A coupling capacity C is inserted in the lead brought to the grid oftube 2, while the potential of the latter with reference to the filamentis determined and governed, for example, by means of a grid-leakresistance It. For the working frequency, the reactance of impedance Lis compensated by the reactance of impedance C. To apply to the plate ofthe tube 1, the necessary direct current potential, the said plate isconnected with a suitable source of potential by way of an impedance Z(resistance or inductance) which will not result in an'appreciable shuntfor the radio frequency current, since the branch or shunt L C forms animpedance sensibly of zero value for this current. It will be noted thatthis coupling arrangement leaves aresidue of amplification for the lowerfrequencies, while it does not transmit the higher frequencies.

' In Fig. 2, I have shown an arrangement for connecting a detector tubeD with a subsequent amplifier tube A. Between the plate of tube D and asuitable point of the supply sources (not shown), there is established aconnection comprising, in series, a reactance coil or choke-coil L atthe plate end and a capacitive reactance C on the source end adjustableas in Fig. 1. The plate of tube D is associated with the positive poleof the plate source by way of a resistance R.

It is possible without great inconvenience to dispose in shunt-to theresistance R an adjustable condenser G with a view to enhance the filteraction, or else a back-feed or tickler coil P in series with thecondenser C A connection or lead associates the grid of tube A with apoint between Land C, said connection comprising the coupling condenserC The potential of the grid of tube A is fixed by curve will have a apulsation 0 R is made equal to p (where p isv the internal resistance ofthe tube), and further Under these conditions, the amplifying shape asshown in Fig. 3. This curve deviates but little from the ordinate l t/2(where it represents the coeflicient of amplification in volts of thetube) for all values of the pulsation being less than al Butitfalls offvery rapidly to zero for the values of the pulsation above (0 The graphs1, II, or Ill are obtained according to whether the resistance R is notshunted; is shunted by the capacity C or shunted by the capacity C inseries with a reactance coil P.

As will be understood without departing from the spii -t underlying thisinvention, it is possible to adopt, according to the end aimed at,values, o er than those hereinbefore indicated, w .h are adapted to andchosen for he conditions under which amplifiers are usually employed inradiophony. The invention and its application will be found particularlyuseful and eilicient in connection with superheterodyne receiver sets,or sets operating on the frequency-changing principle in which appearbeyond the detector not only musical or audio frequency currents, butalso radio frequency currents and currents of an intermediary frequency,of rather large values, which are generated by the action of theoscillator or the local modulator.

There are a. number of evident modifications of the invention that willbe quite apparent to those skilled in the art. F or example, since thecondenser C is designed so as to transmit conveniently audio frequencycurrent, it does not matter whether the condenser C is disposed as shownin Fig. 1, or else is connected after the capacity C in relation to Thatis to say, for example, in parallel relation with the resistance R.

ln lieu of triodes it is also possible to use two-grid tubes ormultiple-electrode tubes, without the herein disclosed coupling meanslosing its special pro )erties. While I have indicated and described asystem for carrying my invention into effect, it will be apparent to oneskilled in the art that my invention is by no means limited to theparticular organization shown and described, but that many modificationsin the circuit arrangements, as well as in the apparatus employed, maybe made without departing from the scope of my invention as set forth inthe appended claims.

What I claim is 1. In an amplifier of lower frequencies resulting fromthe detection of higher frequencies, a coupling between tubes comprisinga series resonant circuit tuned to a definite low frequency andcomprising an inductance in the output circuit of one of the tube-s anda capacity in the input circuit of a following tube, coupling meansbetween said capacity and the grid of the said followingtube, and aresistance included in the circuit comprising the plate of the firstnamed tube and its source of potential, the equivalent impedance of thesaid resistance and the internal resistance of the first tube beingequal to the impedance of the inductance of the series resonant circuitat the said tuned low frequency;

2. In an amplifier low frequencies resulting from the detection ofhigher frequencies, a coupling between tubes comprising a seriesresonant circuit tuned to a definite low frequency and comprising aninductance in the output circuit of a tube and a capacity in the inputcircuit of a following tube, coupling means between said capacity andthe grid of the following tube, and a resistance equal to the internalresistance of the first named tube and included in the circuitcomprising its plate and its .source of potential, this resistance beingtwice the impedance of the inductance of the series resonant circuit atthe said tuned low frequency.

3. In an amplifier of lower frequencies resulting from the detection ofhigher frequencies, a coupling between tubes comprising a seriesresonant circuit tuned to a defiite low frequency and comprising aninductance in the output circuit of one of the tubes a capacity in theinput circuit of a following tube, coupling means between the capacityand the grid of the following tube, and a resistance inserted in thecircuit comprising the plate of the first named tube and its source ofpotential, the equivalent impedance of the said resistance and theinternal resistance of the first tube being equal to the impedance ofthe inductance coil of the series resonant circuit at the said tuned lowfrequency, and the said resistance bein shunted by a variable impedance.

' PAUL BORIAS'.

